Susceptibility measurements on oxygen (${\mathrm{O}}_2$) physisorbed on exfoliated graphite are reported for coverages of up to ρ=3. For 0≤ρ≤1 solidification proceeds by one first-order transition at T’≃25 K associated with a nearly discontinuous decrease of the susceptibility (width about ΔT≃0.5 K) and a nearly δ-like anomaly in the specific heat. For the second and higher coverages solidification spreads over a temperature interval as broad as ΔT≃8 K. This more intricate solidification process is associated with a nearly linear variation of the susceptibility with temperature. We propose to identify this with a two-phase coexistence region corresponding to the θ phase previously proposed on the basis of low-energy electron diffraction measurements. Specific-heat measurements reveal only the low-temperature limit of the solidification process. On the basis of the present measurements we are able to describe some new structures in the ${\mathrm{O}}_2$-graphite phase diagram which allow a more consistent view of all of the hitherto known details.

• Received 29 September 1986

DOI:https://doi.org/10.1103/PhysRevB.35.9809